The use of communication systems in both personal and business day-to-day tasks has become nearly ubiquitous. Both wireline communications networks and wireless communications networks, including the public switched telephone network (PSTN), the Internet, cellular networks, cable transmission systems, local area networks (LANs), metropolitan area networks (MANs), and wide area networks (WANs), are pervasively deployed in modern society and facilitate communication of voice, data, multimedia, etc.
As the use of such communication systems continues to proliferate, the channels through which such communications are conducted become more and more congested. For example, spectrum utilized for wireless communications has become heavily used, leading to limited bandwidth availability for individual devices, interference, poor communication quality, etc.
Various techniques have been adopted to mitigate or avoid interference. For example, frequency division multiple access (FDMA) and time division multiple access (TDMA) techniques have been implemented to facilitate communications by a number of devices simultaneously while mitigating or avoiding interference. However, such techniques are generally not spectrally efficient, in that distinct blocks of spectrum (in frequency and/or time) are reserved for individual communications of a device, whereby those blocks of spectrum are not available for use by other devices. Further compounding the spectral inefficiency of typical communications implementations is the use of frequency division duplexing (FDD) and/or time division duplexing (TDD), whereby the spectrum is further divided such that distinct blocks of spectrum (again, in frequency and/or time) are reserved for uplink communications and other distinct blocks of the spectrum are reserved for downlink communications.
Other techniques for mitigating or avoiding interference have implemented circuitry at a receiving device for attenuating or cancelling interfering signals. For example, some prior attempts have provided active element cancellation through the use of a vector modulator, a radio frequency (RF) multi-tap structure, or nonlinear interference signal cancellation.
The use of a vector modulator for interference cancellation was first introduced by M. E. Knox in “Single antenna full duplex communications using a common carrier,” 2012 IEEE 13th Annual Wireless and Microwave Technology Conference (WAMICON), Florida, 2012: 1-6, the disclosure of which is incorporated herein by reference. The vector modulator measures the power of a received signal and, utilizing a predetermined delay assumption, implements gain control to cancel interference. The use of such a vector modulator has been proposed in various subsequent documents, such as Chinese patent application number CN103580720A, wherein a vector modulator and amplifier are used to cancel a single instance of interference, and United States patent application number US2012/0201153A1, wherein variable delay in a single instance of interference is emulated in a vector modulator by controlling the attenuation of in-phase (I) and quadrature (Q) components of a signal. Such prior vector modulator interference mitigation attempts have, however, only provided for cancelling single path interference. Moreover, the only adaptive control provided by such implementations has been based solely on a receive signal strength indicator (RSSI).
The RF multi-tap structure shown in U.S. Pat. No. 5,691,978 uses multi-tap RF delay and attenuator, providing analog cancellation, with digital adaptive filtering to mitigate interference. The analog cancellation provided by this structure, however, does not provide adaptive control as the delays are predetermined (i.e., relying upon a pre-assumed channel response). Moreover, the digital cancellation does not address nonlinearities in the system, such as may be introduced by active components (e.g., power amplifier (PA)). Similar to the above structure, the RF multi-tap structure shown in US patent applications US2013/0301488A1 and US2014/0219139A1 utilizes pairs of delays and attenuators to cancel multi-path interference in the I and Q signal components. Also like the above RF multi-tap structure, this I/Q RF multi-tap structure utilizes predefined delay and attenuation and does not provide for adaptive control.
Where nonlinear interference signal cancellation has been provided the structure implemented references the signal prior to the active component (e.g., PA) which introduces the nonlinearity and relies upon a non-linier model for the active component. For example, Lauri Anttila, Dani Korpi, Ville Syrjala, Mikko Valkama, “Cancellation of power amplifier induced nonlinear self-interference in full-duplex transceivers <’ 2013 IEEE 47th Asilomar Conference on Signals, Systems, and Computers (ACSSC), 2013, 1193-1198, proposes modeling the nonlinear channel (which is comprised of a nonlinear PA), the linear multi-path channel, and the RF self-interference channel. Similarly, M. Omer, R. Rimini, P. Heidmann, J. S. Kenney, “A compensation scheme to allow full duplex operation in the presence of highly nonlinear microwave components for 4G systems,” 2011 IEEE MTT-S International in Microwave Symposium Digest (MTT), 2011, 1-4, proposes a digital adaptive nonlinear filter to faithfully reconstruct and cancel the PA nonlinearity. It is, however, very difficult to construct an accurate nonlinear model for each active component. Where the magnitude of the interfering signal is significant with respect to the signal of interest (e.g., the magnitude of the interfering signal is as large or larger than the magnitude of the signal of interest), a relatively small amount of modeling error will result in a significant amount of residual interference after application of the interference cancellation.